The invention relates to improvements in the apparatus for the continuous application of a liquid coating to a substrate strip. The invention is applicable to apparatus generally, in which a substrate strip is coated with a coating composition, but was devised primarily for use in the continuous metal coating of steel strip (wherein the liquid coating is molten zinc or molten aluminium zinc alloys) or the continuous coating of steel strip with other liquid coatings such as terne or polymeric paint compositions.
It is commonplace in such apparatus firstly to apply an over thick layer of liquid coating material to the strip at a coating station and then strip surplus material from the over thick layer to the required thickness for the finished coat. The reduction of the over thick layer is generally carried out using a gas jet stripping apparatus.
Gas jet stripping apparatus of the prior art include two elongated nozzles disposed one on either side of the strip""s pass line, which direct sheetlike jets of gas against the respective sides of the thickly coated strip. The two nozzles extend transversely of the strip at right angles to the direction of strip travel. Each gas jet impinges normally or at a certain angle sometimes as large as 30xc2x0 to the strip, and splits into two gas streams flowing over the surface of the strip. One such stream flows in the direction of strip travel and the other flows in the opposite direction. Thus, one of the streams flow counter to the oncoming over thick layer and blows material from the layer back upon itself. The net effect is to prevent all but a thin layer of coating material in close adherence to the substrate strip from travelling with the strip past the nozzles.
For any particular installation, each nozzle is at least as long as the maximum width of strip that may be processed by the installation. Thus, whenever strips of lesser width are being processed, the nozzles extend beyond the edges of the strip. It follows that, beyond the edges of the strip, the end portions of the gas jets meet in opposition, producing a turbulent flow pattern adjacent to the strip edges.
Previously, it has been proposed to modify the traditional gas jet stripping apparatus by the addition of a baffle to a courier plate adjacent each of the strip edges and disposed between the nozzles of the opposed gas jets. The courier plate extends from the vicinity of the strip edge to the maximum strip width and is disposed between the counter flowing gas stream beyond the strip edges.
Each such baffle is a rigid flanged edge having a first portion which extends longitudinally of the strip adjacent a longitudinal edge of the strip, and a second portion, being an extension of the first portion, which diverges away from the edge of the strip in the upstream direction of the strip. The first portion of the flanged edge is spaced a small distance from the longitudinal edge of the strip to allow an unhindered passage of the strip past the baffle.
The effect of the baffle is to contain the stripping counter flowing gas stream (and shield it from the above mentioned turbulence) until such time as the stripping gas stream reaches the second portion of the baffle. In addition, the stripping gas stream adjacent the baffle then tends to follow the diverging second portion of the baffle and thus sweeps across the edge of the strip. In so doing it carries coating material from the margin across the edge and discharges it from the strip as free droplets, so reducing the marginal coating thickness.
To improve the tendency of the stripping gas flow adjacent the strip to follow the diverging second portion of the baffle, there has been proposed in the applicant""s co-pending application No. PCT/AU98/00346, the whole contents of which are incorporated by reference, a flow diversion device including a baffle secured to a plate having a longitudinally extending first portion, a diverging second portion and a gas supply duct for providing gas from an external source to the strip side of the baffle. While the supply of gas to the strip side of the baffle in the region of the second portion of the baffle achieves the desired objectives, the supply of gas from an external source at or above 200 kilopascals above ambient pressure is an additional operating cost.
To improve the operation of the above-mentioned flow diversion device and gas jet stripping apparatus, the applicant has proposed an improvement which is able to utilise the gas from the gas jet nozzles directed at the flow diversion device to supplement or replace the externally sourced gas provided to the strip side of the baffle.
Accordingly the invention provides a flow diversion device in a gas jet stripping apparatus having a pair of opposed gas jet stripping nozzles defining a stripping region, the device being positionable adjacent a moving strip in the stripping region and including a baffle having a first portion extending longitudinally of the strip adjacent the longitudinal edge of the strip and a second portion diverging away from the edge of the strip, a gas outlet for providing gas to the strip side of the baffle, and a gas collection means for redirecting gas from the stripping nozzles to the gas outlet.
When a strip having a width less than the maximum width is used, the flow diversion device is used between the nozzles from the strip edge to the maximum width primarily to reduce the noise and turbulence associated with the meeting of counter flow gas streams. The flow diversion device according to the invention is also able to utilise the gas streams directed at the plate by collecting the gas and preferably directing it towards the gas outlet. The gas collection means preferably includes at least one gas collection conduit having vanes for collecting and diverting gas from said stripping nozzles towards said gas outlet.
The at least one gas collection conduit may be positioned in the stripping zone of the stripping nozzles. Alternatively, the at least one gas collection conduit may be positioned above and/or below the stripping zone of the stripping nozzles to collected gas which has been diverted towards the conduit as a result of contact with the plate.
It is preferable for at least one gas collection conduit to be provided on each side of the plate.
Each of the gas collection conduits is preferably in the form of a conduit with a plurality of vanes to redirect gas from the gas nozzles towards the gas outlet.
In one embodiment, the conduit is positioned along the lowermost edge of the plate, the conduit having vanes on each side of the plate to redirect gas from the stripping nozzles via the gas outlet towards the moving strip.
In many instances, the amount of gas collected and directed towards the gas outlet may be insufficient to enable the baffle of the flow diversion device to operate effectively. Hence, an auxiliary gas supply may be provided to the conduit to supplement the volume of gas passing through the outlet.
In another aspect of the invention, each of the gas supply conduits may be provided with a hood extending up to and preferably contacting the stripping nozzle outlets to essentially enclose the gas passage from the stripping nozzles to the gas outlet. In this way the collection of gas from the stripping nozzles is maximised.
In a preferred form of the invention the baffle of the flow diversion device is secured to the strip side edge of a plate, the plate extending away from the strip between the opposed stripping nozzles.
In another aspect of the invention, there is provided a gas jet stripping apparatus for reducing the thickness of the liquid coating on a moving strip including a pair of opposed gas jet stripping nozzles defining a stripping region and a flow diversion device for positioning between the stripping nozzles in the stripping region and adjacent the moving strip, the flow diversion device including a baffle having a first portion extending longitudinally of the strip adjacent the longitudinal edge of the strip and a second portion diverging away from the edge of the strip, a gas supply outlet for providing gas to the strip side of the baffle, and a gas collection means positioned between the stripping nozzles for redirecting gas from the stripping nozzles to the gas outlet.
The gas jet stripping apparatus according to the invention is able to utilise gas which would normally have been waste gas to reduce or eliminate edge build-up of coating material on the strip.